It was previously shown that the dbl oncogene expressed in SF9 insect cells qualifies as a highly selective guanine nucleotide exchange factor for the human GTP binding protein CDC42Hs. More recently a GDP- dissociation inhibitor (GDI) for CDC42Hs was identified and characterized. This factor stabilizes the GDP-bound state of CDC42Hs and counteracts the action of the dbl oncogene product. It was previously described that the dbl oncogene product shares a region of similarity with CDC24, a Saccharomyces cerevisiae cell cycle gene. Mutations and deletions in this region of similarity completely abolished or drastically reduced the transforming activity of the dbl protein. Ongoing experiments show that mutants lacking transforming activity are also unable to catalyze GDP dissociation from CDC42, indicating that the transforming activity of dbl is associated with its ability to serve as an exchanger for CDC42. Prokaryotic and eukaryotic expression systems have been employed to produce high levels of purified dbl protein. the dbl protein expressed in bacteria was used to generate specific antibodies. The purified bacterially expressed protein is able to induce maturation in oocytes. Purified dbl product expressed in insect cells can be obtained. This purified protein retains exchange activity for the GTP binding protein CDC42Hs.